U.S. Pat. No. 4,244,230 describes a fluidic oscillator flowmeter that is symmetrical about a plane of symmetry. The flowmeter has an inlet in the form of a nozzle feeding a two-dimensional fluid jet into a chamber. Inside the chamber an obstacle has a frontal cavity located in the path of the jet. The oscillation of the jet is transverse relative to the plane of symmetry of the flowmeter, and is accompanied by the formation in the cavity of two vortices, one on either side of the jet. Each vortex is alternately strong and weak, out of phase with the other vortex. During the oscillation the point of impact of the jet sweeps over the wall of the cavity.
Two pressure tappings located on respective sides of the plane of symmetry are connected to one or more pressure sensors in such a manner as to measure the frequency of the oscillation of the jet in the cavity. FIG. 1 shows schematically the electric signal provided by a pressure sensor measuring the difference in pressure between the two pressure tapping points as a function of time t.
Each extreme value has a double peak. The deepest point of the trough located between the two peaks corresponds to the point of impact of the jet passing through its point of maximum deviation.
Thus, if we consider a jet sweeping the cavity, away from the plane of symmetry after passing the region of a pressure tapping, the jet continues its course towards the point of maximum deviation.
After reaching a maximum, the pressure measured at the point of the pressure tapping reduces. On reaching its point of maximum deviation (the bottom of the trough between the peaks), the jet returns towards the plane of symmetry, approaching the pressure tapping, (the second peak corresponds to the passage of the jet through the location of the pressure tapping), then passes through towards the opposite pressure tapping.
On exceeding a suitable selected threshold, the signal S is transformed into square waves by a suitable electronic system. Each square wave corresponds to a volume of fluid which has passed through the flowmeter. The flow can thus be determined by counting the square waves.
The accuracy of such a measurement is equal to one half an oscillation period.
Thus, when the fluid flow is interrupted, at the moment when the oscillation ceases, the exact position of the point of impact of the jet at the bottom of the cavity is unknown, thereby giving rise to uncertainty in the measurement.